In general, a capacitor for smoothing an output voltage and a load such as an electronic control unit are connected between output terminals of a power supply apparatus. For example, if the capacitor becomes disconnected from the output terminals, or if a capacitance of the capacitor is smaller than a predetermined value, ripple components of the output voltage increase, so that operations of the electronic control unit may be affected. Further, if a failure occurs in the electronic control unit as a load, its consumption current (i.e., load current) may exceed a predetermined range.
JP-A-H11-99904 discloses a power supply apparatus including a voltage step-up power supply circuit to charge a capacitor by stepping up a battery voltage, a constant voltage power supply circuit to generate a constant voltage for control, and a series circuit of a switch circuit and a current detection resistor provided in an energization path from output terminals of the voltage step-up power supply circuit to input terminals of the constant voltage power supply circuit. In JP-A-H11-99904, a microcomputer calculates a capacitance of the capacitor based on a change in a voltage of a junction between the switch circuit and the current detection resistor by stopping operations of the voltage step-up power supply circuit after turning ON the switch circuit.
If operations of switching devices in a power supply circuit are stopped under conditions where a capacitor is disconnected from output terminals of the power supply circuit or if a capacitance of the capacitor is extremely smaller than a predetermined value, an output voltage may sharply decrease to 0V, so that a latch-up may occur. Further, if the output voltage sharply decreases to 0V, a delay may occur in an output of a monitor circuit such as a power-on reset circuit. As a result, a puppy supply to a load may be stopped without a voltage reduction signal being outputted. In this case, register values and data values in the load cannot be saved.